1. Field of the Invention
The invention relates to semiconductor processing, and in particular to a system and method for in-situ cleaning for rapid thermal processing systems.
2. Background
In-situ cleaning capability, cleaning the parts within a chamber and its walls while the chamber is in its operating configuration, is an important concept in semiconductor manufacturing. Frequent in-situ cleaning removes unwanted contamination and relieves the need to shut down systems and remove parts for off-system cleaning.
An early example of in-situ cleaning occurs in epitaxy reactors with SiC-coated graphite parts where the cleaning process takes place at temperatures above 1100.degree. C. in a gas environment of H.sub.2 and high concentrations of HCl gas. Another example is the in-situ cleaning of plasma deposition systems in which inert halocarbon feed gases rendered reactive in an electric discharge plasma create radicals containing fluorine and chlorine that remove silicon-based films deposited in the reactor. The reactor temperature in such systems is generally below 600.degree. C. because the plasma supplies the necessary additional energy to create reactive species. Yet another example is the use of trichloroethane in oxygen at nominally 1000.degree. C. to in-situ clean difflusion furnaces to remove trace metals. A highly reactive and poisonous gas, NF.sub.3, has also been used at nominally 900.degree. C. to remove deposits from reactors.
Conventional RTP systems consist of a transparent quartz envelope to provide a controlled atmosphere and an array of radiant heat lamps that shine through the quartz envelope to heat opaque objects within the envelope, such as a semiconductor substrate. The lamp power is turned on after the substrate is inserted and turned off to cool the substrate before it is removed from the quartz envelope. Because the quartz envelope is kept cool, conventional RTP systems are termed cold wall RTP systems.
Some new RTP systems, such as the Aspen RTP System sold commercially by Mattson Technology, typically incorporate one or more SiC-coated graphite heaters, a susceptor or wafer holder made of SiC-coated graphite (solid SiC or solid Si could also be used), an opaque quartz insulator surrounding the heated parts, one or more ceramic and quartz encapsulated thermocouples and a water-cooled aluminum (stainless steel is an alternate choice) vacuum enclosure with view ports, and a robotic load-lock system to control the process atmosphere. Because the substrate is inserted into an already heated enclosure, the systems are termed hot wall RTP systems. The purpose of the SiC coating on the graphite parts is to seal potential metal contamination inside the parts and prevent their migration to the semiconductor substrates.
In its current form, the Aspen RTP system is used for annealing semiconductor substrates at various temperatures from 400.degree. C. to 1100.degree. C. In the course of operating the Aspen RTP system, visible films are deposited on heated parts in proximity to the heater and downstream from the heated parts. Chemical analysis showed that these deposits contained Si, O, C, and N resulting from the purge gases (N.sub.2,O.sub.2, or a combination, possibly with other gases such as NH.sub.3 or H.sub.2) reacting with the heated SiC-coated graphite parts to form volatile compounds that condense on cooler furnace parts.
Some users of the system are concerned that during the annealing of certain semiconductor substrates, out-gassing from the various layers on the substrate could condense in the system and cause problems such as cross-contamination or the creation of particles. Additionally, the build-up of trace metal contamination could allow these contaminants to enter the reactor on other wafers or could migrate from the interior of heated parts to their surface.
The current technology to address contamination in conventional RTP systems is to physically remove the parts and chemically clean them. This is time consuming and results in down-time for the system. An in-situ cleaning process can reduce the down-time of the system, however, in-situ cleaning may damage components of the system such as the SiC-coated graphite or solid SiC parts, the quartz insulators, the ceramic-encapsulated thermocouples, or the metal enclosure.
An in-situ cleaning process should remove unwanted deposits that accumulate either because of out-gassing from semiconductor substrates or because of interaction of the purge gases with the heated parts, remove unwanted metal contamination regardless of its source, be effective at temperatures below 1000IC, not require the use of gases which are reactive in air and/or toxic, and use a cleaning gas that would be acceptable with regard to environmental concerns for global-warming and high altitude ozone-depletion. Additionally, an in-situ cleaning process should not leave residual gas that would affect the properties of substrates processed after the in-situ clean.